The invention is based on a priority application EP07121409.2 which is hereby incorporated by reference.
The present invention relates to a method for radio transmission of data frames from user equipment to a base station in a wireless telecommunication network, a Radio Network Controller “RNC”, a Base Station “BS” and a communication network.
The invention is related to current telecommunication networks such as the Universal Mobile Telecommunications System “UMTS” provide high-speed data communication between wireless devices. UMTS networks introduce new network elements that function according to the specification of the third generation partnership project (3GPP). The network comprises media gateways “MGW”, Radio Network Controllers “RNC” and Base Stations “BS”. The base stations are also referred to as Node-B. Each base station services a particular cell. In radio communications, cell is a small geographic area of radio coverage served by a base station “BS”. A base station “BS” is a wireless communications station installed at a fixed location and used to communicate with user equipment “UE” in a wireless telecommunications system. User equipment “UE” is any device used directly by an end user to communicate. It can be a hand-held telephone, a card in a laptop computer, or other device. The user equipment connects to the base station. A Radio Network Controller “RNC” is the governing element in a radio access network responsible for control of the base stations which are connected to the controller. The Radio Network Controller RNC carries out radio resource management. The Radio Resource Management functionality includes load control, Admission Control, Packet scheduling, Handover control as well as Outer Loop Power Control. The user equipment und base station perform power and quality measurements and send the measurement results to the radio network controller, which performs the handover control and power control based on the received measurement results.
High-Speed Uplink Packet Access “HSUPA” is a third generation mobile telephony protocol. Uplink refers to data transmission form the user equipment, such as a mobile telephone, to the base station. The technical purpose of the HSUPA protocol is to improve the performance of uplink dedicated transport channels, i.e. to increase the capacity and throughput and reduce delay.
The HSUPA uses an uplink enhanced dedicated channel (Enhanced DCH) on which it employs link adaptation methods. It is an enhancement of the uplink dedicated channel (DCH) with focus on more efficient transport of packet switched (PS) data. In packet switching discrete blocks of data (packets) are routed between nodes over data links shared with other packets. In contrast, circuit switched networks establish a dedicated channel between nodes and terminals. The dedicated channel cannot be used by other nodes or terminals.
In the 3GPP UMTS standard of Release 6 the physical channel supports a maximum data rate of up to 5.7 Mbit/s due to turbo coding and code multiplexing of up to two channelization codes of spreading factor two plus two channelization codes of spreading factor four. Two types of transmission-time-intervals (TTI) are supported, 10 ms and 2 ms. Similar to High-Speed Downlink Packet Access (HSDPA), HSUPA supports a MAC-e layer (Media access control layer), which is implemented in the Base Station.
The base station supports fast scheduling. Fast scheduling allows a tighter control of the uplink resources, which allows larger settings of the uplink resource limits. Furthermore soft and softer handovers are supported by the enhanced dedicated channel (E-DCH). The term handover refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. A Soft handover or soft handoff refers to a feature used by the CDMA and WCDMA standards, where a cell phone is simultaneously connected to two or more cells (or cell sectors) during a call. If the sectors are from the same physical cell site (a sectorized site), it is referred to as softer handoff.
The base station may perform HARQ and fast scheduling. HARQ refers to Hybrid ARQ (HARQ), which is a variation of the ARQ error control method. Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which uses acknowledgments and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame. A timeout is a reasonable point in time after the sender sends the data frame; if the sender does not receive an acknowledgment before the timeout, it usually re-transmits the frame until it receives an acknowledgment or exceeds a predefined number of re-transmissions.
In standard ARQ error-detection information (ED) bits are added to data to be transmitted (such as cyclic redundancy check, CRC). In Hybrid ARQ forward error correction (FEC) bits are also added to the existing Error Detection (ED) bits (such as Reed-Solomon code or Turbo code). As a result Hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but in its simplest form this comes at the expense of significantly lower throughput in good signal conditions. There is typically a signal quality cross-over point below which simple Hybrid ARQ is better, and above which basic ARQ is better.
FIG. 1 discloses a standard implementation of a hybrid ARQ. An N-channel parallel HARQ operation is disclosed in FIG. 1. The data packets in FIG. 1 are transferred subsequently as sub-channels in one communication channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel N. Four sub-channels are shown in FIG. 1, namely channels Ch1, Ch2, Ch3 and Ch4. Each channel is connected to its immediately preceding channel by a dotted line, which is either referenced by the sign NACK and ACK. ACK stands for acknowledge. This signal is transmitted to the user equipment, if the received data packet has been correctly transmitted. In this case a new data packet is transmitted across the channel. Alternatively, a not acknowledge NACK signal is sent to the user equipment, if the received data packet contains errors. Consequently, the data packet is retransmitted in the subsequent sub-channel. The maximum number N of HARQ sub-channels for uninterrupted data transfer depends on the transmission-time-interval. In case a transmission time interval of 10 msec is supported by the enhanced dedicated channel, then 4 HARQ sub-channels may occur. Alternatively, if the transmission time interval is equal to 2 msec, then up to 8 HARQ sub-channels are supported.
The round trip time “RTT” for N HARQ sub-channels is equal to N*TTI, wherein TTI is the transmission time interval. Consequently, the higher number of retransmissions introduces additional transmission delay and reduces throughput of the data transfer. If the first transmission fails, then the subsequent HARQ retransmission can be combined with the initial transmission at the base station in order to reduce the required number of retransmissions.
Conventionally, the enhanced dedicated channel E-DCH comprises an inner loop power control between the base station and the user equipment. The inner loop power control controls the power of the data signals from the user equipment to the base station; uplink power. A target for the signal-to-interference ratio of the transmitted data signals is set. If the detected signal-to-interference ratio is lower than the target signal-to-interference ratio, then the transmit power on uplink is increased. Conversely, if the detected signal-to-interference ratio is higher than the target signal-to-interference ratio, then the transmit power is decreased.
Furthermore, the enhanced dedicated channel E-DCH comprises an outer loop power control. The outer loop power control is implemented between the base station and the radio network controller connected to the base station; it controls the target signal-to-interference ratio implemented in the base station. The radio network controller transfers the target signal-to-interference ratio to the base station. The purpose of the outer loop power control is to control the actual number of HARQ retransmissions, which occur in the uplink channel between the user equipment and the base station (Node-B). The outer loop power control increases the target signal-to-interference ratio, if the number of actual HARQ retransmissions exceeds a predetermined number M of target HARQ retransmissions. Conversely, the target signal-to-interference ratio is reduced, if the number of actual HARQ retransmissions is lower than the target number M of HARQ retransmissions. If the packet data unit PDU is correctly received after exactly M target HARQ retransmissions, then the target signal-to-interference ratio is left unchanged. The outer loop power control adjusts the target signal-to-interference ratio to get in average a fixed number M of target HARQ retransmissions.
The state of the art outer and inner loop control algorithm is disadvantageous because it may not provide an optimum throughput. In particular, the user throughput depends in general on the number of users connected to the base station and requiring uplink data transmission to the base station. FIG. 2 depicts a graph of the user throughput in Kbit/s versus the number of users (user equipment) connected to the respective base station. The throughput is depicted for two different fixed target HARQ transmissions. A first graph relates to the achieved throughput in case of 2 target HARQ transmissions (2Tx, i.e. 1 target HARQ retransmission) and a second graph represents the achieved throughput for 1 target HARQ transmission (1Tx, i.e. 0 target HARQ retransmission). As can be seen in FIG. 2, the throughput in case of 1 target HARQ transmission is higher than 2 target HARQ transmissions, if the number of users is lower than 4. But, in case of more than 4 user equipments, the throughput is larger for two target transmissions. The state of the art power control algorithm does not provide an optimum throughput irrespective of the load and number of users in a particular cell.